1. Field of the Invention
This invention relates to the field of information networks, and more particularly relates to a method for discovering preferable routes between two nodes in a network.
2. Description of the Related Art
Today's networks carry vast amounts of information. High bandwidth applications supported by these networks include streaming video, streaming audio, and large aggregations of voice traffic. In the future, these demands are certain to increase. To meet such demands, an increasingly popular alternative is the use of lightwave communications carried over fiber optic cables. The use of lightwave communications provides several benefits, including high bandwidth, ease of installation and capacity for future growth.
The synchronous optical network (SONET) protocol is among those protocols designed to employ an optical infrastructure and is widely employed in voice and data communications networks. SONET is a physical transmission vehicle capable of transmission speeds in the multi-gigabit range, and is defined by a set of electrical as well as optical standards. SONET networks have traditionally been protected from failures by using topologies that support fast restoration in the event of network failures. Their fast restoration time makes most failures transparent to the end-user, which is important in applications such as telephony and other voice communications. Existing schemes rely on techniques such as 1-plus-1 and 1-for-1 topologies that carry active traffic over two separate fibers (line switched) or signals (path switched), and use a protocol (Automatic Protection Switching or APS), or hardware (diverse protection) to detect, propagate and restore failures.
In routing the large amounts of information between the nodes of an optical network, a fast, efficient method for finding the most preferable path through that network is desirable. For example, in the case of voice communications, the failure of a link or node can disrupt a large number of voice circuits. The detection of such faults and the restoration of information flow must often occur very quickly to avoid noticeable interruption of such services. For most telephony implementations, for example, failures must be detected within about 10 ms and restoration must occur within about 50 ms. The short restoration time is critical in supporting applications, such as current telephone networks, that are sensitive to quality of service (QoS) because such detection and restoration times prevent old digital terminals and switches from generating alarms (e.g., initiating Carrier Group Alarms (CGAs)). Such alarms are undesirable because they usually result in dropped calls, causing users down time and aggravation. Restoration times exceeding 10 seconds can lead to timeouts at higher protocol layers, while those that exceed 1 minute can lead to disastrous results for the entire network.
In a SONET network, a failure of a given link results in a loss of signal (LOS) condition at the nodes connected by that link (per Bellcore's recommendations in GR-253 (GR-253: Synchronous Optical Network (SONET) Transport Systems, Common Generic Criteria, Issue 2 [Bellcore, December 1995], included herein by reference, in its entirety and for all purposes)). The LOS condition propagated an Alarm Indication Signal (AIS) downstream, and Remote Defect Indication (RDI) upstream (if the path still exists), and an LOS defect locally. The defect is upgraded to a failure 2.5 seconds later, which causes an alarm to be sent to the Operations System (OS) (per GR-253). When using SONET, the handling of the LOS condition should follow Bellcore's recommendations in GR-253 (e.g., 3 ms following a failure, an LOS defect is detected and restoration should be initiated). This allows nodes to inter-operate, and co-exist, with other network equipment (NE) in the same network. The arrival of the AIS at a node causes the node to send a similar alarm to its neighbor and for that node to send an AIS to its own neighbor, and so on. Under GR-253, each node is allowed a maximum time in which to forward the AIS in order to quickly propagate the indication of a failure.
Thus, the ability to quickly restore network connections is an important requirement in today's networks, especially with regard to providing end-users with acceptable service (e.g., providing telecommunications subscribers with uninterrupted connections). In turn, a method for finding an alternate route with sufficient quality-of-service characteristics in the event of a network failure that is fast and efficient must be provided to enable such quick restoration.